drm/msm: Use struct fb_info.screen_buffer

The fbdev framebuffer is in system memory. Store the address in
the field 'screen_buffer'. Fixes the following sparse warning.

../drivers/gpu/drm/msm/msm_fbdev.c:124:26: warning: incorrect type in assignment (different address spaces)
../drivers/gpu/drm/msm/msm_fbdev.c:124:26:    expected char [noderef] __iomem *screen_base
../drivers/gpu/drm/msm/msm_fbdev.c:124:26:    got void *

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230522191701.13406-1-tzimmermann@suse.de